The present invention relates to a method of displaying composite satellite imagery, and, in particularly, to the display of such images on color display devices limited to 8 bits of color information per pixel.
Composite satellite imagery is the combination of coincident grayshade imagery into a single image in which color, as opposed to grayshade, is a reflection of radiant differences between the original images on a pixel-by-pixel basis. The use of composite imagery techniques can greatly increase the viewable information content of coincident satellite images and lends itself to easy interpretation by a scientist or trained operator. Standard approaches to composite image display require display devices that have a large addressable and viewable color space.
The standard method of generating composite imagery involves the use of display devices that are able to simultaneously display 16 or 24 bits of color. For 8-bit meteorological satellite images, a 16-bit system permits the combination of two images and a 24-bit system permits the combination of three images. The numerical value assigned to a composite image pixel is defined within a 32-bit longword 10 by setting the three lower order bytes, 12, 14 and 16 respectively, to the 8-bit values of the three images for that pixel, i, j, and k, respectively. The first byte 18 is designated as NULL since it is not used when there are three images. This is shown diagrammatically in FIG. 1. For a 16-bit system, either the value of the image byte `k` is set to zero or a two byte word is used consisting of bytes i and j only.
A color lookup table is generated that defines the relationship between the numerical value assigned to a composite pixel and the displayed color of that pixel. It is defined such that the byte value from lowest order to highest defines the intensity of the primary colors red, green and blue, respectively, of the display device. For example when the value of the lowest order byte is zero the red intensity is at 0% and when the value is 255 the red intensity is at 100% of the device's capability. The mechanism by which the color lookup table is defined and loaded is highly hardware and software dependent.
In the resulting composite image, color as opposed to grayness denotes radiant differences (differences in pixel values) between the individual images. See d'Entremont et al., Interpreting Meteorological Satellite Images Using a Color Composite Technique, Bull. Amer. Meteor. Soc., Vol. 68, 1987, pp. 762-768, and d'Entremont et al., Color-composite Image Processing for Multispectral Meteorological Satellite Date, Proceedings of SPIE, October 1987, pp. 96-106 which are incorporated by reference. If the pixel values for each original grayshade image are the same, the composite pixel color is a shade of gray since the intensities of each primary color are equal. If the pixel values for each original image are different, the composite pixel will be a color other than gray, since the intensities of each primary color are unequal. With 256 intensities of red, green, and blue independently available for display the total number of colors available for simultaneous display is over 16 million on a 24-bit system and over 65 thousand on a 16-bit system. This large color space places virtually no restrictions on the quality of the colors in the composite image which exhibits smooth and natural looking color gradations. However, one of the greatest difficulties associated with the display of composite imagery is that 24-bit and 16-bit display devices are expensive and are not common among meteorological researchers and operational forecasting facilities.
Many color display devices, that are otherwise quite powerful, do not support either a 24 or 16-bit magnitude of depth of view in color. This is true of many of the workstations and personal computers used in research and operational environments. Like their 24-bit and 16-bit counterparts, 8-bit devices have a large collection of colors available for display but are only able to display 256 or 8 bits of color at any one time. Thus the present invention seeks to find a method of simulating 24-bit and 16-bit composite imagery on these 8-bit display devices.